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Motion Triggered by Light

A cargo transport system that works at the microscopic level has been devised and constructed by researchers at New York University (NYU), USA. The system is based on self-propelled colloidal hematite particles, which the team calls "dockers".

Jérémie Palacci, Stefano Sacanna, Adrian Vatchinsky, Paul Chaikin, and David Pine of the Center for Soft Matter Research at NYU, can steer the peanut-shaped docker particles on a surface to the vicinity of particles many times their size, "dock" with this cargo, and transport it to another location on the surface either by using a weak magnetic field or nanoscopic tracks on the surface. When the particle cargo reaches a specific location, the dockers can release their payload.

What makes the whole system so astounding, aside from the vast disparity between the size of the docker and its cargo, is that the reversible docking and release process is driven by light.

Manipulating the Microscopic World

The researchers explain that controlled motion and transport of objects are fundamental to our lives from the domestic up to the industrial scale. They add that simple functions are relatively straightforward at the macroscale and indispensable to manufacturing and robotics. However, stepping down from the top to the bottom, at the microscale the world becomes far more complicated. Of course, researchers have been trying for many years to develop synthetic systems that can move microscopic objects for particular tasks.

Emulating the transportation processes inherent in biological systems has been one approach researchers have taken to manipulating the microscopic world, while nanotechnology designed from first principles has also been high on the agenda for at least the best part of two decades. Whatever the approach, success will find applications in medicine and medical research, in areas such as targeted drug delivery, the assembly of micro-robots and other devices, and in microfluidics or microelectromechanical systems (MEMS), which have well-documented potential in many diverse fields from biomedical research and therapies to environmental monitoring and cleanup.

The team explains that their light-activated docking system could provide the means to manipulate microscopic particles in a wide range of applications. The motion and cargo-carrying phenomena observed with the hematite dockers is based on osmotic/phoretic transport triggered by the photocatalytic properties of the hematite immersed in a reservoir of hydrogen peroxide. The light activation, photocatalysis, of the hematite triggers the activity and develops a concentration gradient of hydrogen peroxide, surrounding the particle. The hematite dockers themselves are made in bulk and are simple single-component particles, the team says.

Nanometer-Size Tracks

Writing in the Journal of the American Chemical Society, the team explains that, "The illuminated hematite harvests free energy from the hydrogen peroxide fuel in solution generating an osmotic flow along the substrate. A consequence of this peculiar self-propulsion mechanism is a sensitivity of the active particles to the physical properties of the substrate. This sensitivity can be harnessed to direct the particles along the nanometer-size tracks in a textured substrate". They explain further that, "The docking mechanism is versatile and can be applied to various materials and shapes. This system opens up new possibilities for designing complex micrometer-size factories as well as new biomimetic systems".

The team has demonstrated proof of principle by using spherical silica, polystyrene, and TPM particles of diameters ranging from 1 to 20 µm. The system itself echoes some of nature's transport systems that convert molecular changes into mesoscale or macroscale movement, such as transport of particles in and out of cells and the contraction of muscles. Of course, being light-activated, one might imagine the dockers being exploited in actuators for MEMS or in mechanical movement of components in a robotic or transport system.

In the current work, the team showed that they could direct particles along a straight line, previously textured in the substrate. "It would be interesting to be able to draw more complex patterns, nano-tracks, on the substrate. Then playing with the light, one could pick up a colloid somewhere, bring it to a place with no light, drop off the cargo, and go somewhere else", Palacci told ChemistryViews.org. It might then be possible to assemble quite complicated objects by using this approach with no need for external intervention by an operator. "Fordism brought to the micro scale!" Palacci calls this.

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